AC machines are not inherently fault-tolerant. The primary reason is that the windings of AC machines are closely coupled magnetically, so that a short circuit in one winding has serious effects on adjacent phases. The problem is exacerbated in AC machines having permanent magnets because rotating magnets excite potentially dangerous high currents in any short circuit path. Approaches to enhancing the reliability of AC motor drives and generator systems generally involve the use of two or more AC machines. For example, a common approach is to connect two or more machines on a single shaft. Alternatively, gearing is used to couple the machines together. However, there are weight, volume and cost disadvantages associated with the use of additional machines, thus making such approaches undesirable or even impractical for many applications.
Another approach, which is described in U.S. Pat. No. 4,434,389, issued to Langley et al., is to utilize redundant sets of distributed windings, i.e., windings spread over a number of slots around the air gap periphery. This approach, for machines energized through an inverter, involves dividing a permanent magnet motor into sections, each section comprising one set of magnetically-coupled distributed windings. Each set of windings is energized by a separate commutation circuit, so that the total torque produced is the sum of the torques generated by each set of distributed windings. For each motor section, a command unit detects failures and removes the entire failed motor section from service. Disadvantageously, the close magnetic coupling of the distributed windings makes it necessary to disable the entire set of section windings, even though the fault has developed in only one of these windings. Thus, torque production is reduced by the amount contributed by the entire motor section rather than by the smaller portion delivered by a single winding.
In contrast to AC machines, a switched reluctance (SR) machine is wound using concentrated windings, i.e., windings concentrated on projecting motor poles. As a result, the phase windings of a SR machine are essentially free of any magnetic coupling so that high currents in one winding will not magnetically induce high currents in adjacent phase windings. The present invention utilizes this characteristic magnetic independence of switched reluctance machine phases as the basis for a compact, fault-tolerant motor drive or generator system. Such a fault-tolerant drive can be particularly useful in aerospace applications for which highly reliable drives are necessary.
Switched reluctance machines conventionally have multiple poles on both the stator and the rotor; that is, they are doubly salient. There is a concentrated winding on each of the stator poles, but no windings or magnets on the rotor. Each pair of diametrically opposite stator pole windings is connected in series or parallel to form an independent machine phase winding of the multiphase SR machine. Motoring torque is produced by switching current in each machine phase winding in a predetermined sequence that is synchronized with angular position of the rotor, so that a magnetic force of attraction results between the rotor poles and stator poles that are approaching each other. The current is switched off in each phase before the rotor poles nearest the stator poles of that phase rotate past the aligned position; otherwise, the magnetic force of attraction would produce a negative or braking torque. The torque developed is independent of the direction of current flow, so that unidirectional current pulses synchronized with rotor movement can be applied to the stator pole windings by an inverter using unidirectional current switching elements, such as transistors or thyristors. For use as a generator, the current pulses in each machine phase winding are simply shifted so that current flows when the rotor poles are moving past alignment towards the unaligned position.
A SR motor drive or generator system operates by switching the machine phase currents on and off in synchronism with rotor position. That is, by properly positioning the firing pulses relative to rotor angle, forward or reverse operation and motoring or generating operation can be obtained. Usually, the desired phase current commutation is achieved by feeding back a rotor position signal to a controller from a shaft angle transducer, e.g. an encoder or a resolver. However, in order to reduce size, weight and cost in SR motor drives and generating systems, techniques for indirect rotor position sensing have been developed, thus eliminating the need for a shaft angle transducer. One such technique is disclosed in commonly assigned U.S. Pat. No. 4,772,839, which issued on Sept. 20, 1988 to S. R. MacMinn and P. B. Roemer.
Current regulators are typically employed for controlling phase current amplitudes in a SR machine. There are several types of current regulators. For example, individual low-resistance current shunts may be coupled to each machine phase winding to detect the current level in each phase. The output of each current shunt is connected to a separate voltage comparator. Each comparator is also connected to a separate potentiometer for setting the current limit. Another type of current regulator, which eliminates the need for discrete current sensors, is disclosed in U.S. Pat. No. 4,595,865, issued to T. M. Jahns on June 17, 1986 and assigned to the instant assignee.
Commonly assigned copending U.S. patent application Ser. No. 304,159, filed on Jan. 31, 1989 by G. B. Kliman, S. R. MacMinn and C. M. Stephens, discloses a system for detecting and isolating faults in a SR motor drive, whereby faulted motor phases are deactivated and motor operation is continued. More specifically, this patent application, which is hereby incorporated by reference, describes a SR machine fault management system which detects faults through phase current differential sensing and phase flux differential sensing. In addition, a method is provided for starting the motor when stopped in a "torque dead zone" created by a faulted phase. As used herein, the term "torque dead zone" is a rotor angular position region in which positive motoring torque cannot be produced by any of the intact non-faulted phases. By way of contrast, in a SR generator system, a "voltage output dead zone" is the counterpart to a torque dead zone in a SR motor drive. As used herein the term "voltage output dead zone" is a rotor angular position region in which no voltage output can be generated by any of the intact non-faulted phases.
Although the hereinabove cited patent application advantageously provides a system for isolating and detecting SR machine phase faults, it is desirable to enhance the characteristic independence of SR machine phase windings even further in order to optimize SR machine fault-tolerant performance. In accordance therewith, it is desirable to simplify the fault-tolerant SR machine drive and to prevent the development of "torque dead zones" in motors and "voltage output dead zones" in generators.